Research has consistently found that the month of a child's birth is associated with later outcomes involving health, educational attainment, earnings and mortality. What drives this association remains unclear. Prior explanations for this phenomenon consider social and natural factors (such as compulsory schooling laws or changes in climate) that might affect children born in the winter in particular ways. These explanations often implicitly, and in many cases explicitly, assume that children born at different times of the year are initially similar, but that factors intervene after conception or birth to create differences in outcomes. Our project will consider an alternative and previously-overlooked explanation: those children born throughout the year are not initially similar but are conceived by women with different socioeconomic characteristics. To consider this alternative explanation, this project will have three specific aims. First, this project will use Vital Statistics and census data to document changes in the characteristics of the average woman giving birth throughout the year. Second, this project will use census data to investigate how much of the difference in outcomes (including outcomes related to health, schooling, and earnings) ascribed to season of birth can be explained by simply controlling for the maternal backgrounds of individuals. Third, this project will use Vital Statistics and National Climatic Data Center data to explore weather conditions as a partial explanation for differential fertility outcomes between different groups of women throughout the year. This work will advance research in multiple disciplines, including work on the returns to schooling, work on women's fertility decisions, and work on the relationship between season of birth and later outcomes. This study may provide an important-but-unexplored partial explanation for why outcomes are related to month of birth. This work will also have important implications for a large body of economic research on returns-to- schooling which relies on strong assumptions for why outcomes differ by season of birth;our preliminary studies show that the assumptions made by this body of literature are almost certainly untrue. PUBLIC HEALTH RELEVANCE: This project is relevant to public health as it explores a striking and previously unnoted pattern in births to women during the year. Our study will be, to our knowledge, the first large-scale nationwide U.S. study to explore whether particular groups of women are relatively more likely than other women to conceive or give birth at certain times of year. While women's fertility is itself an important health topic, this project will also advance scientific research on the relationship between season of birth and child health outcomes. Our work proposes an innovative and previously overlooked explanation for why children born in certain times of the year have worse outcomes (including outcomes related to health, schooling, and earnings) than other children. Our work will thus contribute new information to an important aspect of public health-women's fertility outcomes-and provide new explanations for poorly understood phenomena related to child health and wellbeing.